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>Chapter 6. Using Disks and Other Storage Media</TD
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><A
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>6.4. CD-ROMs</A
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><P
>A CD-ROM drive uses an optically read, plastic coated disk.
	The information is recorded on the surface of the disk
	
		<A
NAME="AEN1060"
HREF="#FTN.AEN1060"
>[1]</A
>
		
	in small `holes' aligned along a spiral from the centre to the edge.
	The drive directs a laser beam along the spiral to read the disk.
	When the laser hits a hole, the laser is reflected in one way; when
	it hits smooth surface, it is reflected in another way.  This makes
	it easy to code bits, and therefore information.  The rest is easy, 
	mere mechanics.</P
><P
>CD-ROM drives are slow compared to hard disks.  Whereas a
	typical hard disk will have an average seek time less than 15
	milliseconds, a fast CD-ROM drive can use tenths of a second for
	seeks.  The actual data transfer rate is fairly high at hundreds of
	kilobytes per second.  The slowness means that CD-ROM drives are not
	as pleasant to use as hard disks (some Linux distributions provide
	`live' filesystems on CD-ROMs, making it unnecessary to copy the
	files to the hard disk, making installation easier and saving a lot
	of hard disk space), although it is still possible.  For installing
	new software, CD-ROMs are very good, since maximum speed is not
	essential during installation.</P
><P
>There are several ways to arrange data on a CD-ROM.  The most
	popular one is specified by the international standard ISO 9660.
	This standard specifies a very minimal filesystem, which is even
	more crude than the one MS-DOS uses.  On the other hand, it is so
	minimal that every operating system should be able to map it to its 
	native system.</P
><P
>For normal UNIX use, the ISO 9660 filesystem is not usable, so
	an extension to the standard has been developed, called the Rock
	Ridge extension.  Rock Ridge allows longer filenames, symbolic
	links, and a lot of other goodies, making a CD-ROM look more or less
	like any contemporary UNIX filesystem. Even better, a Rock Ridge
	filesystem is still a valid ISO 9660 filesystem, making it usable by
	non-UNIX systems as well. Linux supports both ISO 9660 and the Rock
	Ridge extensions;  the extensions are recognised and used 
	automatically.</P
><P
>The filesystem is only half the battle, however.  Most CD-ROMs 
	contain data that requires a special program to access, and most of
	these programs do not run under Linux (except, possibly, under
	dosemu, the Linux MS-DOS emulator, or wine, the Windows emulator.
	
	        <A
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>[2]</A
>
	
	There is also VMWare, a commercial product which emulates an
	entire x86 machine in software

                <A
NAME="AEN1069"
HREF="#FTN.AEN1069"
>[3]</A
>)
	
	.</P
><P
>A CD-ROM drive is accessed via the corresponding device file.
	There are several ways to connect a CD-ROM drive to the computer:
	via SCSI, via a sound card, or via EIDE.  The hardware hacking
	needed to do this is outside the scope of this book, but the
	type of connection decides the device file.</P
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><H3
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>Notes</H3
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><A
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>[1]</A
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><P
>That is, the surface inside the disk, on 
		the metal disk inside the plastic coating.</P
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><A
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>[2]</A
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><P
>Ironically perhaps, wine actually stands
		for ``Wine Is Not an Emulator''.  Wine, more strictly, is an
		API (Application Program Interface) replacement. Please see
		the wine documentation at <A
HREF="http://www.winehq.com"
TARGET="_top"
>http://www.winehq.com</A
>
		for more information.</P
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>[3]</A
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><P
>See the VMWare website, <A
HREF="http://www.vmware.com"
TARGET="_top"
>http://www.vmware.com</A
>
		for more information.</P
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